1. Field of the Invention
This invention relates to a system for indicating the mass flow of solids, and particularly provides a system for indicating particulate coal or coal derivative mass flow through a rotating feeder.
2. Description of the Prior Art
In many systems transporting solids or particulate matter, it is important to continuously or intermittently monitor mass flow in order to properly maintain the process control. This is particularly required in the process streams of coal gasification, liquification, and fluidized bed combustion systems. Such systems typically feed coal, or coal derivatives such as char, in particulate form, from lock hoppers through star wheel feeders and, in a transport gas, to the process reactor.
While the mass flow rate of the transport gas, prior to mixing with the particulate matter can be accurately identified with devices such as an orifice, identification of the particulate matter mass flow rate prior and subsequent to mixing with the transport gas, cannot be easily measured with sufficient accuracy. A primary reason for this limitation is that in-line devices, at the operational flow rates, are subjected to a sandblasting type effect which substantially shortens operational life. Additionally, the particulate matter tends to travel at a different flow rate than the transport gas.
Techniques presently used to determine particulate mass flow, upstream of the area of mixing with the transport gas, include lock hopper weight measurement, star wheel feeder rotational speed measurement, and a technique utilizing filtering of lock hopper weight measurement data. Each of these techniques, however, present some undesirable characteristics. Lock hopper weight measurements typically utilize a plurality of strain gauge type load cells which are summed and monitored. These systems are complicated by mechanical restraints, imposed by rigid process system piping and stresses placed upon the hopper by pressurization and thermal expansion, imposing extraneous stresses on the gauges. Designs can be provided which guarantee a free floating lock hopper and load cell system, and which provide accurate indications, but only with substantial complexity.
With utilization of a star wheel feeder, although the volume of the feeder pockets is known, the pockets are not uniformly filled. Particle size, moisture content and other variables affect the mass transfer rate for a given revolution rate. Additionally, fine material tends to be retained for a period of time within the feeder pockets and the surrounding casing as a result of pressure differentials, thus providing inaccurate mass flow indications.
More recently proposed are systems for measuring flow of particulate coal mixed with a transport gas which utilized temperature measurements and a heat balance. While such systems represent an improvement in the art, additional improvements can be made. For example, such measurements are limited to flow of solid material in a transport gas, and require input from multiple thermal monitors.
It is therefore desirable to provide substantially on-line systems for determining particulate mass flow rate which alleviate the discussed limitations. It is further desirable to provide systems which are simple, reliable and which take advantage of existing technology and components.